The present invention relates generally to the production of laminated glass sheets and, more specifically, to an improved apparatus for prepressing anti-lacerative or laceration shield type glazings particularly adapted for use in automotive vehicles.
In the early stages of the automobile industry, single sheets of ordinary glass were employed as windshields. As it became evident that this type of windshield presented a considerable safety hazard, the single sheets of ordinary glass were replaced with single sheets of heat treated or tempered glass. Thereafter, as laminated safety glass was developed to reduce the severity of lacerative injuries, its use in automotive windshields greatly increased until today, when almost all automotive windshields are constructed of some type of laminated glass.
Typically, laminated glass of the type utilized in vehicle windshields consists of two sheets of glass bonded together with a thin plastic interlayer, such as a sheet of polyvinyl butyral, for example. In the event of an impact on a laminated glass windshield sufficient to break the glass, the plastic interlayer functions to bind the glass fragments together, thus reducing the risk of injury to a driver or passenger as a result of flying glass or contact with the windshield. Further developments with this type of laminated glass, such as those disclosed in U.S. Pat. No. 3,231,461, have resulted in laminated windshields with improved penetration resistance. Consequently, with the ever growing recognition of the necessity for increased safety precautions, continuing efforts have been and are still being made to appreciably reduce the injury producing potential of automobile windshields.
Recently, it has been found that the addition of a second plastic layer bonded to the inner glass surface of the laminated windshield further increases the safety effectiveness of the windshield. This second plastic layer has typically been termed a protective laceration inhibiting shield since it has been found that the additional plastic layer will appreciably reduce the number and severity of lacerative injuries to persons thrown against the windshield under all impact conditions. Further, it has been found that the laceration shield, when produced under certain conditions of manufacture, improves the ability of the laminated windshield to decelerate movement of a person thrown against the windshield, while also increasing the penetration resistance of the windshield as compared to conventional laminated windshields. Also, the laceration shield reduces the amount of flying glass and thus the injury to car occupants as a result of objects that may be thrown against the windshield from overpasses or elsewhere outside the vehicle.
An example of an automotive windshield which incorporates, as part of its laminated structure, a protective laceration shield bonded to its inboard glass surface is disclosed in U.S. Pat. No. 4,242,403. In this patent, the laceration shield includes a penetration resisting multi-layer body consisting of an inner layer of relatively soft, extensible plastic material such as polyvinyl butyral, for example, which is adhered to the inborad surface of the windshield, an intermediate layer of more durable plastic such as polyester, and an outer coating of an abrasion resistant material.
While the effectiveness of a laminated windshield having a laceration shield is obvious, very few vehicles utilize such a windshield. The chief reason for this limited use has been the difficulty experienced in trying to manufacture a windshield with a laceration shield on a production basis. In the automotive industry, the standard for windshields is very high, especially as to optical qualities, and it has been very difficult to manufacture a windshield having a laceration shield which is free of optical defects. Even when the individual sheets of the laminated assembly are free from optical defects before bonding them together, it is difficult to join them and preserve the optical qualities.
One method for applying a plastic layer to one surface of a single sheet of glass is disclosed in U.S. Pat. No. 3,806,387. In this method, a sheet of glass, a layer of adhesive, and a layer of thin transparent plastic sheeting are assembled in a stack to produce an assembly for subsequent lamination. A second sheet of glass (called a glass cover or foaming sheet) conforming to the configuration of the sheet of glass in the stacked assembly is then placed on top of the platic sheet. The surface of the glass forming sheet which is placed adjacent the plastic sheet is coated with a demolding agent to prevent any adhesion between the glass forming sheet and plastic sheet. Typically, the glass forming sheet is bent on the same form as the glass sheet of the laminated assembly. Next, the spaces between the individual laminae are evacuated and the assembly is positioned in an autoclave. The autoclave applies pressure to the exterior surfaces of the assembly while heating the assembly to a temperature which causes bonding between the glass sheet and the plastic sheet. After the assembly is removed from the autoclave, the forming sheet can be removed from the stack.
One cause of defects in laminated glass structures is air trapped between the layers. It has been found to reduce such defects if, prior to treatment in the autoclave, the assembly is pressed, or subjected to vacuum at its edges, or both. The preliminary pressing, or "prepressing" operation, is typically performed by an apparatus having upper and lower series of pressing or nipper rollers arranged in axially parallel, tangentially contacting relation. Generally, the rollers are mounted in a cage or frame adapted to be swung arcuately from an assembly receiving position in an assembly discharge position. It has also been found advantageous to apply a peripheral evacuation chamber less to thickness than the assembly to which it is attached enabling the evacuation chamber and assembly to pass between the prepressing rollers. Such an apparatus is disclosed in U.S. Pat. No. 4,040,888.
One of the problems with the prior art evacuation chamber was that it required a thin flexible tape of air impervious material to seal the evacuation chamber to the laminated assembly. Then, when the prepressing was completed, the tape had to be stripped from the laminated assembly. All of this taping required time and manual labor and left a sticky residue of adhesive on both surfaces of the laminated glazing. Furthermore, the use of the rigid tubes made processing curved assemblies extremely difficult and required the nipper rollers to be positioned a fixed distance apart less than that which would enable the most efficient prepressing operation.